The present invention relates to a non-cyanide electroless gold plating solution and a process for electroless gold plating.
The process for producing a semiconductor device includes a step mounting a solder ball 16 as an external connecting terminal on a pad 14 constituting an end part of a wiring pattern 12 made of copper and formed on one surface of a substrate 10, as shown in FIG. 1.
At the formation of such a solder ball 16 on the pad 14 made of copper, a resinous mask layer 18 such as a solder resist is first formed on one surface of the substrate 10 and then a concave portion 20 where a pad surface is exposed at the bottom is formed in the mask layer 18 by laser or the like.
Then, in the concave portion 20, after a metal layer 22 made of Ni—P is formed as an underlying layer by electroless nickel plating, the substrate 10 is dipped in an electroless gold plating solution in order to form a gold layer 24 thinner than the metal layer 22 on the upper surface of the metal layer 22 by immersion gold plating.
Thereafter, by placing a solder ball on the gold layer 24 and reflowing it, gold is diffused into the solder and an alloy of Ni—P of the metal layer 22 with the solder is formed, thereby firmly fixing the solder ball 16 as an external connecting terminal.
As the electroless gold plating solution in which the substrate 10 is dipped at the time of immersion gold plating for forming the gold layer, there has been hitherto employed a cyanide-containing electroless gold plating solution to which a cyanide complex such as KAu(CN)2 is added as a gold source.
However, since the cyanide-containing electroless gold plating solution is generally used in an alkaline region, the resinous mask layer 18 such as a solder resist is apt to be eroded when the substrate 10, one surface of which is covered with the mask layer 18, is dipped in the cyanide-containing electroless gold plating solution.
Moreover, with regard to the cyanide-containing electroless gold plating solution, special care should be taken in its handling owing to its toxicity and also a special operation is necessary for detoxifying it in the case of its disposal.
Furthermore, the cyanide-containing electroless gold plating solution has a problem that a reaction with the underlying Ni—P metal layer 22 (displacement reaction) is apt to proceed excessively and pinholes remain in the underlying Ni—P metal layer, that is, Ni erosion generates.
Instead of such a conventional cyanide-containing electroless gold plating solution, a non-cyanide electroless gold plating solution free from a cyanide compound has been proposed in Hyomen Gijutsu (Surface Finishing), vol. 52, No. 5, 2001 (pages 410 to 413), for example.
In such a non-cyanide electroless gold plating solution, a gold sulfite complex is added as a gold source in order to stabilize a gold ion in the electroless gold plating solution.
However, since such a non-cyanide electroless gold plating solution is still used in an alkaline region, the resinous mask layer 18 such as a solder resist is apt to be eroded when the substrate 10, one surface of which is covered with the mask layer 18, is dipped in the non-cyanide electroless gold plating solution.
Furthermore, the non-cyanide electroless gold plating solution containing a gold sulfite complex as a gold source is extremely low in stability as described in Hyomen Gijutsu (Surface Finishing), vol. 46, No. 9, 1995 (pages 775-777). Therefore, frequency of replacement of the electroless gold plating solution increases.
In order to solve the problems of the non-cyanide electroless gold plating solution, there has been reported a cyanide-containing electroless gold plating solution to which sodium mercaptoethanesulfonate (MES) is added (Patent Document 1).
According to the non-cyanide electroless gold plating solution, since it is possible to use it in a region of pH 7 or lower, there are advantages that a resinous mask layer such as a solder resist is not eroded and also solution stability is satisfactory.
[Patent Document 1] US 2006/0062927 A1
However, with regard to the above non-cyanide electroless gold plating solution to which MES is added, no problem arises in the case that the underlying metal layer is a so-called low-phosphorus type Ni—P plated metal layer of a phosphorus content of about 1 to 4% by weight. But, in the case that a so-called medium-phosphorus type plated metal layer of a phosphorus content of about 6 to 9% by weight, there is a problem that the underlying metal layer is eroded and roughened by the non-cyanide electroless gold plating solution. Therefore, there is a problem that satisfactory bonding reliability of solder cannot be attained.